The amount of municipal waste in the past decades has grown at an accelerated rate. Today's products are no longer made to endure but to be disposable. The quality of these products is dictated by the optimal economic life, which is of course determined by its manufacturer. In addition, consumers are becoming less tolerant of old and used products than ever before. As a result, products are thrown out at an unprecedented faster rate. The consequence is that we have overflowing landfill sites.
Historically, the solution is to let the waste accumulate in landfill sites until the site are full, and then a new site is selected. This is a quick and cheap option, and it could be even more efficient if more people followed the “Reduce, Reuse and Recycle” attitude. However, land is becoming more scarce and expensive as the population grows. In addition, the accumulation of landfill sites poses many problems such as the production of hazardous wastes. Two main hazardous byproducts of landfill sites are leachate, a toxic liquid, and landfill gas, a flammable greenhouse gas. Leachate is a strong and unpleasant smelling liquid waste produced from rainwater, underground water and other moisture the landfill waste may contain that soaked is with the garbage. It has a very high chemical oxygen demand making it very difficult and expensive to treat. The landfill gas is a product of anaerobic biodegradation, and it mainly contains methane and carbon dioxide, both of them are main green house gas. The methane is highly flammable and can cause fires and explosions. These byproducts must be addressed with better solution.
To solve these problems, scientists and engineers have developed three primary technologies to process MSW. However, each of them has their own limitations:
1. The sanitary municipal landfill process. This process has combined the leachate collection and treatment system, landfill gas collection system, and in addition, some are also equipped with a landfill gas torch, and thermal or thermal to electricity system as well. The sanitary landfill process also requires clean soil cover daily to prevent further pollution.
Generally speaking, a sanitary landfill process uses a single cell with life cycle between 20-25 years. After this period, the landfill site is sealed and left for another 30 years before it becoming reusable. This is because the biodegradation of the organic trash under the sealed field is still continuing; a cycle that usually takes upwards of 20-30 years or longer, depending on the local climate—especially the temperature and amount of the rainfall. During this cycle, fires and explosions are likely to take place. It may also contribute to ground subsidence. Therefore this site cannot be used effectively for an extensive amount of time, even after the site has been sealed. In this period, the extensive leachate and landfill gas collection and treatment may still be required. Thus, this process creates land resource waste, and continually treatment expenses in order to avoid serious pollution of the underground water source and the air.
2. Municipal solid incineration. In this process, the MSW is incinerated to generate heat or heat to electricity. However, the cost is extremely high (100-168 USD per tonne of MSW). In addition, it produces high carcinogenic substance-dioxins and can cause serious air pollution. Furthermore, the incinerated ash must be treated by a landfill process. The incineration also produces massive amount of greenhouse gases. As the recycling program is gaining popularity, the thermal value of the MSW to be incinerated will decrease further because the plastic, paper, fabric, rubber etc will be sorted out and sent for recycling, the cost of incineration will be even higher since a larger amount of additional fuel is required.
3. Municipal solid waste compost technology. In this process, the MSW must be well sorted and the un-recyclable organic trash is used to produce fertilizer. This process produces extensive amount of greenhouse gases. Moreover, since the high-temperature fermentation is very costly, the medium and the low temperature fermentations process are often being employed. As a result, this process requires much more time, and the medium and low temperature fermentation process is unable to thoroughly extinguish the bacteria and viruses, so it may lead to contamination of farms, crops and the human food chain.